The 2009 Swine Flu pandemic was caused by a new strain of H1N1 influenza A virus that had not been recognized previously in pigs or humans, although six of its eight gene segments were similar to ones previously detected in triple reassortant swine influenza viruses in pigs in North America. The strain represents a quadruple reassortment of two swine strains, one human strain, and one avian strain of influenza. The largest proportion of genes comes from swine influenza viruses (30.6 percent from North American swine influenza strains, 17.5 percent from Eurasian swine influenza strains), followed by North American avian influenza strains (34.4 percent) and human influenza strains (17.5 percent). Pigs play an important role in interspecies transmission of influenza virus. Susceptible pig cells possess receptors for both avian (alpha 2-3-linked sialic acids) and human influenza strains (alpha 2-6-linked sialic acids), which allow for the reassortment of influenza virus genes from different species if a pig cell is infected with more than one strain.
A typical feature of newly emergent pandemic influenza strains is that severe infection occurs disproportionately in individuals who are not at the extremes of age. In contrast, seasonal influenza is more likely to cause severe disease in infants, young children, and elderly individuals. The virus can be spread amongst humans from direct contact which can occur through coughing, sneezing or contamination of hands and surfaces. The severity of symptoms is highly variable, although with most people suffering only relatively mild symptoms. Patients are considered contagious for up to a week after the onset of symptoms but children may be contagious for longer periods of time. Influenza virus is present in respiratory secretions of infected persons. As a result, influenza virus can be transmitted through sneezing and coughing via large-particle droplets. Transmission via contact with surfaces that have been contaminated with respiratory droplets or by aerosolized small-particle droplets may also occur, although these modes of transmission have not been proven. In addition to respiratory secretions, certain other bodily fluids (e.g., diarrheal stool) should also be considered potentially infectious.
The signs and symptoms of influenza caused by pandemic H1N1 influenza A virus are similar to those of seasonal influenza, although gastrointestinal manifestations appear to be more common with pandemic H1N1 influenza A. The severity appears to be less than what was observed during the influenza pandemic of 1918 to 1919. The most common clinical findings of the 2009 H1N1 influenza A pandemic have been fever, cough, sore throat, malaise and headache; vomiting and diarrhea have also been common, both of which are unusual features of seasonal influenza. Other frequent findings have included chills, myalgias, and arthralgias. Both leukocytosis and leucopenia have been observed among hospitalized patients in Mexico, many hospitalized patients have had leucopenia, elevated aminotransferases, elevated lactate dehydrogenase, and elevated creatinine phosphokinase. Some patients have also had renal insufficiency.
To establish the diagnosis of pandemic H1N1 influenza A, an upper respiratory sample (nasopharyngeal swab, nasal swab, throat swab, combined oropharyngeal/nasopharyngeal swab, or nasal aspirate) should be collected. A confirmed case of pandemic H1N1 influenza A is defined as an individual with an ILI with laboratory-confirmed pandemic H1N1 influenza A virus detection by real-time reverse transcriptase (RT)-PCR or culture.
Recommendations on whom to test may differ by state or community. Not all individuals with suspected pandemic H1N1 influenza A need to have the diagnosis confirmed, particularly if the illness is mild or the person resides in the area of confirmed cases. The recommended test to confirm the diagnosis of pandemic H1N1 influenza A virus is real-time reverse transcriptase (RT)-PCR for influenza A, B, H1, and H3 (He J, Bose M E, Beck E T, Fan J, Tiwari S, Metallo J: Rapid multiplex reverse transcription-PCR typing of influenza A and B virus, and subtyping of influenza A virus into H1, 2, 3, 5, 7, 9, N1 (human), N1 (animal), N2, and N7, including typing of novel swine origin influenza A (H1N1) virus, during the 2009 outbreak in Milwaukee, Wis. J. Clin. Microbiol. 2009, 47:2772-2778). However, in some regions of the country, RT-PCR is performed only when the results will substantially impact clinical management or when there is a recognized public health benefit. The strain of H1N1 influenza A virus associated with the 2009 pandemic tests positive for influenza A and negative for H1 and H3 by real-time RT-PCR. Isolation of pandemic H1N1 influenza A virus using culture is diagnostic, but culture is usually too slow to help guide clinical management. A negative viral culture does not exclude pandemic H1N1 influenza A infection. Clinicians may consider using rapid influenza antigen tests as part of their evaluation of patients suspected of having pandemic H1N1 influenza A, but results should be interpreted with caution (Dong H, Zhang Y, Xiong H: Detection of human novel influenza A (H1N1) viruses using multi-fluorescent real-time RT-PCR. Virus Res 2009, 147(1):85-90).